Salmonella typhimurium tester strain TA-100 is being used in a study of the reaction rates of styrene oxide isomers. It has been noted previously that the (plus) and (minus) styrene oxide isomers showed different rates of conjugation with glutathione. To date, we have shown that the (minus) isomer produces the same level of mutagenic response as the racemic mixture (1:1) over a timed course of exposure. The addition of uninduced rat liver soluble enzymes (cytosol) and glutathione to the preincubation mix, decreases the mutagenic response for both the (negative) isomer and the racemic mixture. Additional testing performed on styrene glycol and styrene-glutathione conjugate at concentrations up to 10 times the styrene oxide levels, showed no mutagenic or toxic responses.